Yea I'm not sure the cause. Thing is Huang Zhong + Zhao Yun can be put in the billeting. Only characters that can't be in there are Zhang Fei / Bao, Guan Yu / Xing, Zhu Ge Liang and Liu Bei.

One theory I've heard is that if you have a character die and resurrect them, they then go last the rest of their time used in the game. I've never seen this myself, but maybe it's possible. I do know though that when I said Ji Ling went last, even after Rebel Forces, he hadn't died for me. So who knows, really.